Tunable lasers have been known for some time. FIG. 1 illustrates a simple tunable laser configuration 10. It comprises a tunable solid state active medium or a dye solution 12 which is excited by a suitable laser 14 emitting an excitation laser beam .lambda..sub.L. A resonator is formed by a pair of mirrors 16, 18, of which mirror 16 has a high reflectance while mirror 18 is partially transmitting. A stimulated laser beam .lambda..sub.tunable is output through partially transmitting mirror 18. In this illustration excitation beam .lambda..sub.L and tunable laser beam .lambda..sub.tunable are at right angles, a configuration known in the art as transverse pumping.
In the transverse pumping arrangement, the population inversion in the active medium is non-uniform along the path of the excitation laser beam since the beam is attenuated in the active medium. This gives rise to large diffraction loss and large beam divergence.
A better configuration for laser pumped tunable lasers is the longitudinal pumping arrangement 20 of FIG. 2. Again, this arrangement comprises an active medium 22, an excitation laser 24, and two spaced mirrors 26, 28 that define a resonator cavity. However, in this arrangement an excitation beam .lambda..sub.L passes through one of the resonator cavity mirrors 26 of the tunable laser cavity. While entry mirror 26 transmits the excitation radiation, it almost totally reflects the tunable laser emission. The second mirror 28 is a partially transmitting mirror that serves as an "exit" or output mirror that permits emergence of a tunable laser output .lambda..sub.tunable. This configuration leads to greater spatial uniformity of the tunable laser beam.
The output of the lasers described in FIGS. 1 and 2 above has a rather broad bandwidth because tunable solid state crystals or dyes generally have wide fluorescence spectra. The broadband fluorescence can be used advantageously to tune a laser, i.e., to readily obtain highly monochromatic laser emission of any given frequency within the fluorescence spectrum. Fine tuning of the laser wavelength and simultaneous attainment of narrow linewidth can be achieved by using wavelength-selective elements in the resonator cavity.
One method which is commonly used for achieving a small spectral linewidth employs one or more birefringent filters and/or etalons in the resonator cavity. One drawback of this method is that alignment becomes very complicated because of multiple elements in the cavity. Furthermore, each element introduced into the cavity produces loss of the output power.
Another method makes use of devices for spatial wavelength separation. FIG. 3 illustrates a resonator 30 comprising an active medium 32, a mirror 36 and a rotatable grating 38. The grating is set at the Littrow mount position and autoreflects radiation of the desired wavelength back to the active medium. This class of resonator is also not free of problems. First, the grating may be damaged by a high power incident beam. Second, in this configuration only a small area of the grating can be illuminated resulting in a poor spectral resolution. Both of these problems are solved in resonator 40 of FIG. 4 by the use of beam expanding optics 39 with the same components as in resonator 30. However, the introduction of beam-expanding optics often causes undesirable reflection losses, complicates optical alignment, and increases the sensitivity to thermal damage.
Furthermore, most of the above mentioned lasers operates at a relatively low repetition rate of about 10 Hz limiting the capability of data acquisition and reducing the signal to noise ratio. Moreover, bulkiness of the laser configuration gives rise to long cavity lengths L. As a result, the free spectral range C/2L (C is the velocity of light) which is the reciprocal of the time it takes a light beam to make a round trip between the cavity mirrors is correspondingly small; and undesirable mode hopping can result.
Thus there is a need for a spectrally narrow linewidth tunable laser having a high output repetition rate that does not compromise other properties.